Matt Clarke answers some of the most frequently asked questions on pH.

What is pH?

The pH shows how acidic or alkaline water is, and is a measure of the amount of hydrogen and hydroxyl ions present in the water.

How does the pH scale work?

The pH scale extends from 0 to 14 with 0 being extremely acidic and 14 being extremely alkaline. A pH of 7 is neutral, that is, neither acidic nor alkaline.

The critical thing to remember about the pH scale is that it is logarithmic. This means there is a tenfold difference in each unit. A pH of 8 is 10 times more alkaline than a pH of 7, and a pH of 6 is 10 times more acidic than a pH of 7. So what appear to be small changes in the water chemistry to us are much larger in chemical terms and can have a dramatic effect upon the fish.

How can I measure pH?

You can buy a simple liquid or tablet test kit for 5-10, or a more accurate meter or computer for 50-200.

Test kits come in several forms designed for measuring high range pH (more than 7), low range pH (less than 7) and a broader pH range. Broad range kits are not very accurate and only provide a rough approximation. You can get special kits designed for use in saltwater.

Do some fish need water of a specific pH?

Those from acid waters, like the Amazon and forest pools in West Africa, are called acidophiles and prefer a pH below 7. Fish from alkaline waters, like marines or Malawian or Tanganyikan cichlids, are alkalophiles and prefer a pH over 7, preferably around 8. Both groups are intolerant of the wrong pH.

My tapwater is pH 8. Should I make the water softer and more acidic for my Neons?

While acidophilic fishes prefer soft, acidic conditions, if you live in a hard water area, your shop will probably have acclimatised them to hard, alkaline water. While some shops lower their pH, most keep their community fish in adjusted tapwater.

If you want to keep trickier fishes like Discus, Rams or certain anabantoids, it might be necessary to change the pH and hardness before buying fish. Always ask your retailer about their water when getting new stock for your tank.

If you want to breed fish from softwater areas, you'll need to reduce the pH and hardness to get them to spawn. There may be a link to the chemistry of the water and the number of healthy eggs or fry produced.

What makes pH go down?

The main reason for a drop in pH is carbon dioxide. This gas dissolves in water to form carbonic acid, which gives rise to the hydrogen ions that reduce pH. CO2 is produced by the respiration of plants, bacteria and fish. Other processes can also contribute to a drop in pH, including nitrification, methane fermentation and sulphide oxidation.

This means that if you have lots of fish, lots of plants, or a dirty substrate or clogged undergravel filter, you are most at risk of pH drops. The water that leaves some denitrators can also be very acidic.

What makes pH go up?

Many rocks and gravels sold for use in the tropical freshwater tank contain minerals that increase the pH and hardness. If you don't want the pH to increase, ask your dealer for inert substrates and decor.

Untreated concrete around a pond can increase the pH to dangerous levels, often leading to the loss of fish. Always treat nearby concrete with a pond-sealant paint such as G4.

Photosynthesis can also increase pH. If the tank or pond gets lots of sunlight and contains loads of plants or algae, the pH can get very high in the day.

What is the best way to increase my pH?

If you have soft, acidic tapwater and you wish to keep fish that like hard, alkaline water, the best way to increase the pH and hardness is to use aragonite. This is a calcium-rich mineral and boosts and maintains a high pH and hardness. Unlike coral sand, coral gravel or dolomite chips, aragonite starts to dissolve at a pH of about 7.8 rather than about 8.

Why can't I reduce my pH?

It is difficult to reduce the pH of your water if the KH (carbonate hardness) is too high. The KH provides a buffer to acids and prevents them from decreasing the pH. Some carbonate hardness is important, otherwise the pH will drop as acids are produced in the tank.

Should I add a chemical pH increaser or reducer to the tank to adjust my pH?

Adding liquid or powdered products to increase or decrease pH is rarely a permanent solution. The majority of aquaria have a fairly high KH which provides a buffer to acids, for example.

Adding these products could cause potentially stressful fluctuations in pH. I avoid using them where possible. I would recommend avoiding them unless you really know what effect they are having.

What is the safest way to reduce my pH?

Because a high KH reduces your chances of keeping the pH down, start off with RO water which has a low mineral content. Add a pH-reducing acid, such as API's pH Down, until you reach the desired mark. Replenish hardness with a mineral supplement.

I'm adding CO2 for my plants but it affects my pH. Why?

Carbon dioxide forms carbonic acid when added to water, which reduces the pH. Plants take in CO2 in the day to produce oxygen, but at night the process is reversed. This means that the pH can get very low in the morning and rise during the day. Turn off your CO2 supply at night and add an airstone if the pH drops too low.